How can I set a time between slides to create an automatic slideshow with beamer?

You can use the \transduration command for this (page 142 of the manual):

\transduration<⟨overlay specification⟩>{⟨number of seconds⟩} In full screen mode, show the slide for ⟨number of seconds⟩. If zero is specified, the slide is shown as short as possible. This can be used to create interesting pseudo-animations. Example: \transduration<2>{1}

Example:

\documentclass{beamer}

\begin{document}

\begin{frame}
\transduration{0.75}
This is my first frame
\end{frame}

\begin{frame}
\transduration{0.75}
This is my second frame
\end{frame}

\begin{frame}
\transduration{0.75}
This is my third frame
\end{frame}

\end{document}

output - I use convert -verbose -delay 50 -loop 0 -density 300 file.pdf file.gif to make gif.

May be you can try the Impress!ve PDF viewer, which gives numerous functionalities dedicated to presentations (and of course, fine timing of slides).

Note: this is not a "pure" beamer solution since to have the full usage of impressive power you will have to define a script associated with your PDF file. Well, in other words, you can also consider that this is a PDF-origin independent solution.